The invention relates to a cold-rolled screw which is intended for screwing into plastic, has a self-tapping thread and of which the thread turns are configured approximately in the manner of knife blades in cross section. The invention also relates to such a screw in which the free end of its shank forms a guide for screwing the screw into a bore.
Such screws are widely used. As an example, you are referred to the screw according to German Patent 27 54 870. When such a screw is screwed into plastic, the latter is displaced by the thread turns penetrating into the plastic, for which purpose the thread cross section has to provide a sufficient clearance, to be precise a spacing between a screw-receiving bore and the core diameter of the screws. In the case of the known screw, this is also achieved in that the thread root has a narrowed fraction. This means that the plastic material displaced by the thread has to cover the distance between that region of the thread turn which has penetrated into the plastic and the thread root. In this case, the displaced plastic material loses the direct, intimate contact with the undisplaced plastic material, this reducing its capability of contributing to the drawing-out force. The drawing-out force is to be understood as that force which is necessary for pulling out the screwed-in screw.
Furthermore, EP Patent 133 773 describes a screw which is intended for screwing into plastic, has a self-tapping thread and in the case of which, for the purpose of increasing the release torque, the thread flanks, in axial section are rectilinear on one side and are provided with an outwardly directed inflection on the other side. With this thread configuration, it is not possible, during displacement of the plastic, to counteract the loss of contact between displaced and undisplaced plastic. Furthermore, EP Patent 476 831 likewise presents a non-symmetrical configuration of the thread of a screw which is intended for screwing into plastic and has a self-tapping thread, it being the case that this configuration results in the thread turns, in axial section, being rectilinear on one side and being allowed to run in arculate form into the thread root on the other side, the flank angle of the thread thus increasing constantly in the region of the arc. This is intended to improve the displacement of the plastic material with the effect of increasing the drawing-out force.
You are additionally referred to German Utility Model 79 25 469, which discloses a welding tip which is provided with a thread and in the case of which the thread serves for pressing a workpiece, which is provided with an opening, onto the welding tip and for securing the workpiece in that sharp-edged thread crests press into the material of the workpiece and secure the latter against a drawing-out force. In the case of the known welding tip, the thread is provided with sharp edges in that a peripheral phase [sic] is pressed laterally onto the thread crest, said phase [sic] merging with a sharp-edged formation into one thread flank. This necessarily gives a particularly large flank angle of approximately 75xc2x0 on the thread crest. This configuration means-that the thread provided on the known welding tip is not suitable for a screw which is intended for screwing into plastic and has a self-tapping thread since screws for screwing into plastic only ensure a sufficient penetration depth of the thread turns when the cross section of the thread turns is designed approximately in the manner of knife blades (see abovementioned German Patent 27 54 870).
European Patent Application 102 605 presents a thread form which is similar to the configuration of German Utility Model 79 25 469, is intended for a wood screw, but does not have a sharp-edged thread crest, and thus does not have any properties which could make it suitable for screwing into plastic.
The object of the invention is for the self-tapping thread of a cold-rolled screw to be configured, in terms of the axial section through the thread turns, such that the plastic material which is displaced when the screw is screwed into plastic can flow away in a particularly favourable manner.
This is achieved according to the invention in that a flank surface of the thread turns has an inwardly directed inflection approximately in the central third and forms an outer flank angle of approximately 30xc2x0 (between the inflection and the thread crest) and an inner flank angle (between the inflection and the thread root), it being the case that the inner flank angle, on average, is essentially equal to or greater than ⅓ of the outer flank angle.
On account of the inwardly oriented inflection of the flank surface of the thread turns, the plastic material which is displaced by the thread turns is displaced into the region of the inner flank angle, it being the case that this region, on account of the inwardly oriented inflection and the clearance which thus extends relatively far outwards, directly adjoins the displaced plastic material protruding convexly from the bore wall without any material accumulation, with the result that the displaced plastic material, on account of the short displacement distance, is only heated to a minimal extent and remains in direct intimate contact with the undisplaced, and thus unimpaired plastic material. This means that this displaced plastic material, which largely maintains its properties, in the region of the inner flank angle can counteract the drawing-out force and thus makes a significant contribution to the loading capacity of the relevant screw connection.
It is possible for the inflection to be provided in each case just on one flank surface of a thread turn, in which case, over the entire thread, the inflection is arranged on one side throughout. However, it is also possible for the inflection to be provided in each case on both flank halves of a thread turn. The arrangement of the inflection on just one flank surface depends on the plastic material into which the relevant screw is to be screwed. If the plastic material is heat-sensitive, then the inflection is advantageously arranged on that flank surface which is directed away from the screw head (rear flank). In this case, there is a deformation of plastic material essentially on the side of the rear flank where the deformed plastic material flows into the clearance provided by the inflection, with the result that on the flank surface which is directed towards the screw head (load flank) there is largely unheated plastic material for absorbing the forces acting on the screw. If, on the other hand, the plastic material is temperature-resistant (as is the case, in particular, as a result of glass-fibre reinforcement), then the inflection is advantageously arranged on the load flank since, in this case, plastic material which is displaced when the screw is screwed in is accumulated to a considerable extent in front of the load flank, said plastic material, on account of its quantity and, if appropriate, as a result of the reinforcement provided by its filler, then withstands particularly well the loading to which it is subjected by the screw.
The outer flank angle may bound the thread turns symmetrically or non-symmetrically in axial section. A symmetrical boundary is favourable for the production of the rolling jaws necessary for the cold-rolling operation; a non-symmetrical boundary gives increased drawing-out forces in the case of certain plastic materials.
The configuration of the flank surfaces in the region of the inner flank angle may be selected such that the flank surfaces run rectilinearly or in a concavely curved manner from the inflection in axial section. The selection of this configuration depends on the plastic into which the screw is to be screwed.
As far as the configuration of the inner flank angle of the thread turns is concerned, it is also possible for said angle to bound the thread turns symmetrically or non-symmetrically in axial section. The selection of this boundary likewise depends essentially on the plastic into which the screw is to be screwed.
In order to increase the stability of the thread turns and to improve the material flow during the production of the screw by cold rolling, the screw is expediently configured such that the flank surfaces in the region of the thread base merge into the thread root in the form of a widening of the thread base, the inner flank angle being increased in the process. In this case, the widening forms a flank angle which, on average, is greater than the outer flank angle. It is possible here for the widening to run rectilinearly in axial section, but it is also possible for the widening to run in a concavely curved manner in axial section.
It is expedient for the configuration of the screw and the plastic material and the diameter of the screw-receiving bore to be coordinated with one another, for which purpose it is possible to use a series of different shapings, as described above by way of example. It is to be ensured here that, when the screw has been screwed in, the inflection in the flank surfaces is enclosed by plastic material. The inflection is expediently located at a diameter which is approximately equal to or greater than the bore diameter. For this purpose, a position of the inflection approximately in the centre of the flank surfaces has proven favourable. In this case it is possible, also for the purpose of facilitating the screw production, for the inner flank angle to be selected such that the latter is equal to or greater than half the outer flank angle, provided the resulting clearance is of a sufficient size for the displaced plastic to flow in.
A further known screw which is widely used is disclosed in German Patent 31 17 624. The known screw has a thread which tapers conically to a point, this being intended to facilitate the operation of screwing the screw into a workpiece. This is because, when the screw is screwed in, there is a transition from the lesser thread diameter to the full thread diameter, as a result of which the screw can be screwed in first of all with a small amount of force, whereupon, as the screwing-in operation continues, the full widening of the cut thread is to be made possible with a relatively low torque. The disadvantage of this screw configuration is that during widening of the thread, along the conical region of the thread, the workpiece is subjected to a considerable radial pressure, which cannot always be readily absorbed by the relevant material. It is thus also known to have thread forms where the thread end can terminate with decreasing height of the thread turn, the thread vertex finally reaching the external diameter of the shank, which corresponds to the thread root. Such a configuration is disclosed in German Patent 40 03 374. According to British Patent 976 849, a further configuration of a screw with a self-tapping thread consists in that axial grooves are made in the region of the thread end, with an external diameter which is smaller than the load-bearing thread region, said axial grooves allowing sharp-edged teeth to project from the thread as in the case of a screw tap. This configuration of the screw means that the latter is itself capable of cutting, with chip formation, a thread of reduced external diameter, which it then definitively widened by the load-bearing thread region.
The invention also has the object of providing a screw which corresponds to the definition given in the introduction and which performs the thread-cutting operation with the avoidance of chip formation and forms the thread in the material of the relevant workpiece such that the material is, as it were, divided by the thread of the screw and pushed away in a controlled manner to produce a thread groove.
This is achieved according to the invention in that the thread end is formed by an end surface which truncates the thread turn of the thread end, is directed obliquely with respect to the cross section of the screw and has a cutting edge and a base line, which continues the thread-base edge between one thread flank and the lateral surface of the shank, it being the case that the base line runs at an angle xcex2, which differs considerably from 90xc2x0, with respect to the direction of slope (lead angle xcex1) of the thread and the cutting edge emerges into the thread vertex.
When the free shank end of the thread is introduced into the bore of a workpiece, the leading end of the thread turn presses into the material of the workpiece by way of its end surface, it being the case that, on account of the angle xcex2 maintained by the base line and of the resulting inflection of the cutting edge with respect to the thread vertex, the end surface cuts open the material of the workpiece, to be precise with the cutting edge, which merges into the thread vertex. This results in a displacement of the material, which is pushed away laterally essentially just on the side of the end surface, i.e. just on the side of one thread flank, while such a displacement does not take place in practice on the side of the other thread flank. As the threads turn cuts in, its region which adjoins the end surface obtains the necessary clearance for the rest of the screwing-in operation of the thread, the material of the workpiece pressing flush against the thread-turn flank which is located opposite the end surface. Depending on the application, this has a favourable effect on the pulling-out forces. This is also basically attributable to the fact that, by virtue of the division of the material of the workpiece, the material on the side of one thread flank, as a result of material displacement being prevented there, virtually fully maintains its structure and, consequently, its inner strength. The cutting edge of the end surface thus acts like a cutter.
The end surface may advantageously be configured essentially as a triangle, of which the base, running between its base points, is formed by the base line and the vertex coincides with the thread vertex.
As has been described above, the triangular end surface is positioned obliquely with respect to the cross section of the screw. It is possible to select different oblique positions, that is to say either such that the vertex of the triangle follows or precedes the base line in the screwing-in direction of the screw. The first case gives greater design freedom as far as the end surface is concerned since in this case the [sic] extends, as it were, on the outside of the last thread turn. In the case of the vertex preceding the base line, the end surface forms in the end of the thread something of an undercut, which cannot be of just any depth. Said end surface is expediently of planar configuration. However, it is also possible for the end surface to be of curved, to be precise convex or concave, configuration if this results in a more favourable push-away action of the material. In this case, it is also possible for the curvature to be spherical.
A further possible configuration of the end surface consists in allowing the latter to extend in a helical configuration from one thread flank, to be precise such that the end surface then runs flat into the lateral surface of the shank.
In order to facilitate production of the screw, the end surface is expediently provided with a considerable length, in particular the end surface extends over at least xc2xc of a thread turn. A correspondingly long cutting edge is also obtained in this case, this making it easier to cut open the material.
There are basically two possibilities for the oblique position (angle xcex2) of the end surface with respect to the direction of slope of the thread. On the one hand, it is possible to select the angle xcex2 such that it is considerably less than 90xc2x0 (less than 75xc2x0).
In this case, the end surface subjects the material of the workpiece to a pressure which is oriented in the direction of the screw end, that is to say there is a compression of the material on the so-called rear flank, which is located opposite the load flank. The load flank of the thread is that flank which, when the screw has pulling-out forces acting on it, absorbs these forces. These pulling-out forces run axially from the screw end to the screw head and cause the screw to be pulled out of the workpiece in this direction. The accumulation of displaced material on the rear flank, which is equivalent to the material of the workpiece on the side of the load flank remaining virtually unimpaired by the screw being screwed in, gives more resistance to the pulling-out forces, in particular when the screw is screwed into plastic, than deformed material on the side of the load flank, so that this alignment of the end surface, as tests have shown, results in greater pulling-out forces than does the opposite alignment of the end surface. If, on the other hand, the angle xcex2 is selected such that it is considerably greater than 90xc2x0 (greater than 105xc2x0), then the material coming into contact with the end surface is pushed away from the screw end by said end surface, that is to say there is a material accumulation on the load flank.
The end surface can be configured by various production processes, in particular by cutting or non-cutting forming.